1. Field of the Invention
The present invention relates to a deflecting/scanning apparatus used for an image forming apparatus such as a laser printer or a laser facsimile apparatus.
2. Related Background Art
In a deflecting/scanning apparatus used for an image forming apparatus such as a laser printer or a laser facsimile apparatus, as shown in FIGS. 1 and 2, laser light emitted from a light source unit 101 is condensed into a beam of light by a cylindrical lens 102. The light beam is then deflected/scanned by a rotating polygon mirror 103 in a predetermined direction (Y-axis direction) perpendicular to a direction (Z-axis direction) along the rotation axis of the rotating polygon mirror 103 to be focused onto a photosensitive drum 105 via an imaging lens system 104 constituted by a spherical lens 104a and a toric lens 104b. The light beam focused onto the photosensitive drum 105 forms an electrostatic latent image by main scanning in the Y-axis direction upon rotation of the rotating polygon mirror 103 and by subscanning in the Z-axis direction upon rotation of the photosensitive drum 105.
Scanning light L.sub.0 from the rotating polygon mirror 103 is separated downward by a BD mirror 106a at one end of the scanning plane (X-Y plane) in the Y-axis direction to be incident on a BD sensor 106b. The light is then converted into a scanning start signal and transmitted to a semiconductor laser in the light source unit 101. Upon reception of the scanning start signal, the semiconductor laser starts write modulation.
The light source unit 101, the cylindrical lens 102, the rotating polygon mirror 103, the imaging lens system 104, the BD mirror 106a, and the BD sensor 106b are mounted on the side and bottom walls of an optical box 107. The photosensitive drum 105 is disposed outside the optical box 107, and a window 108 is formed in a side wall of the optical box 107. The scanning light L.sub.0 emerges from the optical box 107 toward the photosensitive drum 105 via the window 108. The upper opening of the optical box 107 is covered with a lid 109 shown in only FIG. 2.
The optical box 107 and the lid 109 respectively have a partition wall 107a (see FIG. 1) and a protruding portion 109a (see FIG. 2) protruding toward the spherical lens 104a. The internal space of the optical box 107 is divided, by the partition wall 107a and the protruding portion 109a, into a space portion 110a around the rotating polygon mirror 103 and a drive unit 103a therefor, and a space portion 110b facing the window 108. This structure prevents a large amount of outer air from entering the optical box 107 owing to a suction force generated upon rotation of the rotating polygon mirror 103.
According to the above conventional technique, however, the partition wall of the optical box and the protruding portion on the lid alone cannot sufficiently reduce the amount of outer air flowing around the rotating polygon mirror. For this reason, dust entering the optical box together with outer air accumulate on and around the motor for the rotating polygon mirror. Every time the motor rotates, such dust flies into the air to considerably contaminate the reflecting surfaces of the rotating polygon mirror, resulting in a great deterioration in the optical performance of the deflecting/scanning apparatus.
In addition, as shown in FIG. 2, lenses such as the spherical lens and the toric lens are fixed on a plurality of base seats 107b protruding upward from the bottom wall of the optical box 107 with an adhesive or the like. For example, a gap of about 1 mm is left between the bottom surface of each of the lenses 104a and 104b and the bottom wall of the optical box 107 on which no base seat 107b is disposed. For this reason, when the rotating polygon mirror 103 rotates at a high speed, air currents are drawn in via the above gaps, and dust carried by the air currents contaminates the reflecting surfaces of the rotating polygon mirror. Color toner or the like containing no magnetic component tends to float in the image forming apparatus. Color toner particles or the like in the above air currents therefore pass through even a very small gap. Consequently, in a color image forming apparatus, contamination on the reflecting surfaces of the polygon mirror tends to progress more quickly. As the reflecting surfaces of the rotating polygon mirror are quickly contaminated, the reflecting mirrors of the polygon mirror must be cleaned or replaced frequently, resulting in a considerable increase in maintenance cost.